Different size images in each row of twentyfourteen grid?

I’m just wondering if anyone knows a way to make the size of images in each row of the grid different in the Twentyfourteen theme? I’m working locally right now so I can’t post a link, but what I’m hoping to do is use a 3-column layout but have only one long image in the first row followed by 3 images in the second row. Anyone have suggestions? Thanks